Field of Science

New Late Triassic Sauropodomorph, Chromogisaurus novasi from Argentina

Ezcurra, M. D. 2010. A new early dinosaur (Saurischia: Sauropodomorpha) from the Late Triassic of Argentina: a reassessment of dinosaur origin and phylogeny. Journal of Systematic Palaeontology 8: 371-425.

Abstract - It was traditionally thought that the oldest known dinosaur assemblages were not diverse, and that their early diversification and numerical dominance over other tetrapods occurred during the latest Triassic. However, new evidence gathered from the lower levels of the Ischigualasto Fm. of Argentina challenges this view. New dinosaur remains are described from this stratigraphical unit, including the new species Chromogisaurus novasi. This taxon is distinguished from other basal dinosauriforms by the presence of proximal caudals without median notch separating the postzygapophyses, femoral lateral surface with deep and large fossa immediately below the trochanteric shelf, and metatarsal II with strongly dorsoventrally asymmetric distal condyles. A phylogenetic analysis found Chromogisaurus to lie at the base of Sauropodomorpha, as a member of Guaibasauridae, an early branch of basal sauropodomorphs composed of Guaibasaurus, Agnosphitys, Panphagia, Saturnalia and Chromogisaurus. Such an affinity is for the first time suggested for Guaibasaurus, whereas Panphagia is not recovered as the most basal sauropodomorph. Furthermore, Chromogisaurus is consistently located as more closely related to Saturnalia than to any other dinosaur. Thus, the Saturnalia + Chromogisaurus clade is named here as the new subfamily Saturnaliinae. In addition, Eoraptor is found to be the sister-taxon of Neotheropoda, and herrerasaurids to be non-eusaurischian saurischians. The new evidence presented here demonstrates that dinosaurs first appeared in the fossil record as a diverse group, although they were a numerically minor component of faunas in which they occur. Accordingly, the early increase of dinosaur diversity and their numerical dominance over other terrestrial tetrapods were diachronous processes, with the latter preceded by a period of low abundance but high diversity.

Besides the information provided in the abstract here are a couple of other tidbits:

1) Diagnosis: the holotype material is very fragmentary, but Chromogisaurus novasi is diagnosed by "the following combination of characteristics (autapomorphies∗): proximal caudals without median notch separating the postzygapophyses; ilium with strongly posteriorly developed postacetabular process; incipiently perforated acetabulum; a femoral lateral surface with deep and large fossa immediately below the trochanteric shelf;∗ and a metatarsal II with strongly dorsoventrally asymmetric distal condyles" (Ezcurra, 2010:374).

2) Etymology:  The generic name is a combination of the Greek "chroma" (color) and "gi" (ground or land) for the Painted Valley where the specimen was collected.

3) Clades: This paper provides definitions for the newly recovered clades Guaibasauridae and Saturnaliinae.

4) Chindesaurus: The paper contains discussion on the phylogenetic position of Chindesaurus bryansmalli from the Chinle Formation.  This study (378 characters) finds Chindesaurus as eithr a basal theropod or as a non-eusaurischian saurischian.

5) Photos: There is a nice comparative figure (photos) of the skulls of Herrerasaurus, Eoraptor, and Zupaysaurus.

I'm looking forward to reading the rest of this....

2 comments:

  1. Excellent to see this out, though does anyone else find it funny/annoying that we have better descriptions of Chromogisaurus, Guaibasaurus, Panphagia, etc. than we do for Eoraptor that was named 17 years ago?

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  2. With the caveat, that I haven't seen the paper.... I'm intrigued that _Eoraptor_ (as a theropod) is more derived than herrerasaurids (non-eusurischian saurischians). It was my impression that _Eoraptor_ lacked the grasping/raking manus and intramandibular joint of theropods, whereas _Herrerasaurus_ had both these features.

    But as I said, I haven't seen the paper. Although I'd love to...my email address is tijawi@yahoo.com, if some kind soul wants to send me a pdf. ;-)

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